DC-AC converter, and method for supplying AC power

ABSTRACT

First and second semiconductor switches which are activated alternately are provided between ends of a primary winding and a common potential point, wherein a DC power supply voltage is supplied to a center tap. An electric current flowing into a load is fed back to thereby subject the semiconductor switches to PWM control. Series circuits consisting of capacitors and semiconductor switches are connected between the center tap of the primary winding and the ends of the same. The semiconductor switches are activated in synchronism with the first and second semiconductor switches, thereby preventing occurrence of an anomalous high voltage, which would otherwise be caused at the time of switching operation.

This application is a Continuation of application Ser. No. 11/340,396,filed Jan. 26, 2006, which is a Continuation of Ser. No. 10/806,262filed Mar. 22, 20004, which applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a DC-AC converter (hereinafter referred to asan “inverter”) for producing an AC voltage to be used for driving a loadfrom a DC power source such as a mains adapter, which is an accessory ofan electrical appliance, or a bat tery, as well as to an AC power supplymethod.

2. Description of the Related Art

A cold cathode fluorescent lamp (CCFL) has come into use as a back lightsource for a liquid-crystal monitor of a notebook PC or a liquid-crystaldisplay of a liquid-crystal TV receiver and so on. In general, the CCFLmight be of higher efficiency and longer life use than those of anordinary hot cathode fluorescent lamp. A filament usually provided inthe hot cathode fluorescent lamp is omitted from the CCFL.

Start-up and activation of the CCFL require a high AC voltage. Forinstance, a start-up voltage is about 1,000 volts, and an operatingvoltage of the CCFL is about 600 volts. Such a high AC voltage isproduced from a DC power source of a laptop computer or a liquid-crystalTV receiver through use of an inverter.

In conventional arts, Royer circuit has been known for an inverter useof a CCFL. The Royer circuit is formed from a saturable magnetic coretransformer, a control transistor, and other circuit elements. The Royercircuit can perform a self-oscillation by means of non-linearpermeability of the saturable magnetic core transformer and a non-linearcurrent gain characteristic of the control transistor. However, theRoyer circuit itself does not need any external clock or driver circuit.

The Royer circuit is basically a constant-voltage inverter. Therefore,when fluctuation arises in an input voltage or a load current, the Royercircuit cannot maintain a constant output voltage. Consequently, aregulator is required for supplying the power to the Royer circuit. Inthis regard, it was difficult for an inverter using the Royer circuit tobe down-sized and achieve a efficient low power conversion.

Among other related arts, there has been proposed an inverter using acenter-tap-type transformer, in which the transformer has a primarywinding which supplies a DC voltage to a center tap, and a secondarywinding for outputting an AC voltage such as disclosed in theInternational Patent Publications of No. WO 99/34651 or Japanese PatentPublication Hei 6-14556.

An inverter disclosed in WO 99/34651 has a center tap of the primarywinding provided with a DC voltage, and semiconductor switches providedbetween respective ends of the primary winding and the ground so thatthe semiconductor switches are alternately operated in On-state andOff-state. Further, the inverter is provided with a PWM controller forperforming a PWM control so as to supply DC voltage to the inverter. Thepower supplied from the inverter to the load can be controlled by meansof said DC current control performed by the PWM controller.

An inverter of the JP Hei 6-14556 comprises a primary winding having aDC power source connected to a center tap; a secondary winding foroutputting an AC voltage; a booster transformer having a tertiarywinding for feedback purpose; a resonance capacitor connected betweenboth ends of the primary winding of the booster transformer toconstitute an LC resonance circuit between the capacitor and theinductance of the primary winding; a pair of semiconductor switches, oneend of each semiconductor switch being connected to the other end of theresonance capacitor, the remaining end of each semiconductor switchbeing connected to the earth, and the semiconductor switches beingalternately operated On-Off states in accordance with a voltage outputfrom the tertiary winding; and a variable inductance connected to theinside of an LC resonance circuit. The voltage output from the inverteris controlled by means of controlling the inductance of the variableinductor.

The inverter using the conventional Royer circuit has difficulty to bedown-sized and suffers a problem of low conversion efficiency. Theinverter of WO 99/34651 additionally requires a PWM controller forperforming the PWM control so as to supply DC voltage to the inverter,of which overall structure of the DC-AC converter ends up beingcomplicated, and difficulty still remains in its down-sizing. As for theinverter of JP Hei 6-14556, since it has a variable inductor connectedto the inside of the LC resonance circuit such that an output voltagecan be controlled by controlling the inductance of the inductor, thestructure of the inverter becomes complicated and difficulty ofdown-sizing is still unresolved.

SUMMARY OF THE INVENTION

It is an objects of this invention to provide an inverter for producingAC voltage from the DC power source to be used for driving a load with asimple configuration with a fine control of the power supply to the loadby using a transformer having a primary winding with a center tap towhich DC voltage is supplied.

An inverter of the first aspect in this invention is characterized bycomprising: a transformer having a primary winding with a center tap,and at least one secondary winding, in which the center tap is connectedto a first potential point of a DC power source; a first semiconductorswitch which is connected between one end of the primary winding and asecond potential point of the DC power source and causes an electriccurrent to flow through the primary winding in a first direction; asecond semiconductor switch which is connected between the other end ofthe primary winding and the second potential point and causes anelectric current to flow through the primary winding in a seconddirection; a first capacitor and a third semiconductor switch, which areconnected in series between the other end of the primary winding and thecenter tap; a second capacitor and a fourth semiconductor switch, whichare connected in series between the one end of the primary winding andthe center tap; a current detection circuit to be used for detecting anelectric current which flows into a load connected to the secondarywinding; a pulse width modulation circuit which produces a pulse widthmodulation signal by comparing a feedback signal formed on the basis ofan electric current detected by the current detection circuit with atriangular wave signal train; and a logic circuit to be used foroutputting a switch drive signal and which produces, on the basis of thepulse width modulation signal, a first switch drive signal to be usedfor turning on the first semiconductor switch, a second switch drivesignal to be used for turning on the second semiconductor switch, athird switch drive signal to be used for turning on the thirdsemiconductor switch, and a fourth switch drive signal to be used forturning on the fourth semiconductor switch, wherein the first throughfourth switch drive signals are produced at timings of an off-stateperiod where the first through fourth semiconductor switches are turnedoff, said timings being produced during interval periods such that afirst semiconductor switch group, comprising the first semiconductorswitch and the third semiconductor switch, and a second semiconductorswitch group, comprising the second semiconductor switch and the fourthsemiconductor switch, are alternately made on-state synchronously withtriangular wave signals of the triangular wave signal train.

An inverter of the second aspect in this invention is characterized inthat the first and third semiconductor switches are turned on at everyother peak of the triangular wave signal of the triangular wave signaltrain and remain on-sate until an immediately-subsequent triangular wavesignal becomes equal to the feedback signal; and further wherein thesecond and fourth semiconductor switches are turned on at every otherpeak of the triangular wave signal of the triangular wave signal train,which is different from that of being generated when the first and thirdsemiconductor switches are turned on, and remain on-state until animmediately-subsequent triangular wave signal becomes equal to thefeedback singal.

An inverter of the third aspect in this invention is characterized inthat the first semiconductor switch is turned on at every other peak ofthe triangular wave signal of the triangular wave signal train andremains on-state until an immediately-subsequent triangular wave signalbecomes equal to the feedback signal; the second semiconductor switch isturned on at every other peak of the triangular wave signal of thetriangular wave signal train, which is different from that of beinggenerated when the first semiconductor switch is turned on, and remainson-state until an immediately-subsequent triangular wave signal becomesequal to the feedback signal; the third semiconductor switch is turnedon at a timing before the first semiconductor switch is turned on, butafter lapse of a predetermined period of time since the secondsemiconductor switch completes the on-sate period thereof and remainson-state during a period in which the first semiconductor switch remainson-state; and

the fourth semiconductor switch is turned-on at a timing before thesecond semiconductor switch is turned on, but after lapse of apredetermined period of time since the first semiconductor switchcompletes the on-state period thereof and remains on-state during aperiod in which the second semiconductor switch remains on-state.

An inverter of the fourth aspect in this invention is characterized inthat the first through fourth semiconductor switches are MOSfield-effect transistors.

The fifth aspect in this invention is characterized in that a burstcontrol signal in the form of a pulse train, which allows a ratio ofon-state period and off-state period to be controlled, whereby the firstthrough fourth switch drive signals can be produced or stopped.

The sixth aspect in this invention is characterized in that a powersupply voltage of a DC power source is converted, to a load connected toa secondary winding of a transformer, the transformer having a primarywinding with a center tap, and at least one secondary winding, themethod comprising the operations of: connecting the center tap to afirst potential point of the DC power source; connecting a firstsemiconductor switch for causing an electric current to flow through theprimary winding in a first direction between one end of the primarywinding and a second potential point of the DC power source; connectinga second semiconductor switch for causing an electric current to flowthrough the primary winding in a second direction between the other endof the primary winding and the second potential point of the DC powersource; connecting a first capacitor and a third semiconductor switch inseries between the other end of the primary winding and the center tap;connecting a second capacitor and a fourth semiconductor switch inseries between the one end of the primary winding and the center tap;detecting an electric current flowing through the load connected to thesecondary winding by means of a current detection circuit;

producing a feedback signal on the basis of the electric currentdetected by the current detection circuit and comparing the feedbacksignal with a triangular wave signal train, thereby producing a pulsewidth modulation signal; and producing, on the basis of the pulse widthmodulation signal, a first switch drive signal to be used for turning onthe first semiconductor switch, a second switch drive signal to be usedfor turning on the second semiconductor switch, a third switch drivesignal to be used for turning on the third semiconductor switch, and afourth switch drive signal to be used for turning on the fourthsemiconductor switch; whereby producing the first through fourth switchdrive signals at timings of an off-state period where the first throughfourth semiconductor switches are turned off, said timings beingproduced during interval periods such that a first semiconductor switchgroup, comprising the first semiconductor switch and the thirdsemiconductor switch, and a second semiconductor switch group,comprising the second semiconductor switch and the fourth semiconductorswitch, are alternately made on-sate synchronously with triangular wavesignals of the triangular wave signal train.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall block diagram of an inverter according to a firstembodiment of the invention;

FIG. 2 is an internal block diagram of an inverter control IC for use inFIG. 1;

FIG. 3 is a timing chart of a first example of the inverter shown inFIG. 1;

FIG. 4 is a view showing operating states of the inverter at timingsshown in FIG. 3;

FIG. 5 is a waveform diagram for describing operation of the inventionin comparison with a related-art inverter;

FIG. 6 is a timing chart of a second example of the inverter shown inFIG. 1;

FIG. 7 is a view showing operating states of the inverter at timingsshown in FIG. 6;

FIG. 8 is a view showing another example in which the main circuitconfiguration of the inverter shown in FIG. 1 has been changed;

FIG. 9 is a view showing still another example in which the main circuitconfiguration of the inverter shown in FIG. 1 has been changed; and

FIG. 10 is a view showing yet another example in which the main circuitconfiguration of the inverter shown in FIG. 1 has been changed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

By reference to the drawings, there will be described a mode forrealizing an inverter which produces, from a DC power source, an ACvoltage to be used for driving a load and implementing an AC powersupply method, both pertaining to the invention.

FIG. 1 is a view showing the entire configuration of an inverter of theembodiment of the invention which performs PWM control operation throughuse of an insulating transformer having a primary winding with a centertap and a secondary winding and through use of semiconductor switchingcircuits. FIG. 2 is a view showing the internal configuration of an ICfor controlling the inverter.

In FIG. 1, a transformer TR is an insulating transformer having aprimary winding 107 and a secondary winding 108 for supplying AC powerto a load, wherein the primary winding 107 has a center tap T, oneterminal A (hereinafter called a “first terminal”), and the otherterminal B (hereinafter called a “second terminal”). A DC supply voltageVCC, being supplied from a battery power source BAT, is supplied to thecenter tap T of the transformer TR where said VCC is a voltage from theground which is serving as a common potential point.

An n-type MOSFET (hereinafter abbreviated “NMOS”) 101 serving as a firstsemiconductor switch is a switch to be used for forming a current pathin a first direction on the primary winding 107 of the transformer TR.On the other hand, NMOS 102 serving as a second semiconductor switch isa switch to be used for forming a current path in a second direction onthe primary winding 107 of the transformer TR. Thus the NMOS 101 and theNMOS 102 are alternately activated, which generate an alternatingcurrent to flow into the primary winding 107 of the transformer TR.

A DC circuit, which consists of a first capacitor 105 and a p-typeMOSFET (hereinafter abbreviated “PMOS”) 103 serving as a thirdsemiconductor switch, is connected between the center tap T of theprimary winding 107 and the second terminal B. The PMOS 103 is basicallycontrolled so as to be turning on synchronously to the NMOS 101. In thesame manner, DC circuit, which consists of a second capacitor 106 and aPMOS 104 serving as a fourth semiconductor switch, is connected betweenthe center tap T of the primary winding 107 and the first terminal A.The PMOS 104 is basically controlled to be turning on synchronously withthe NMOS 102.

The NMOS 101, the NMOS 102, the PMOS 103, and the AMOS 104 each have abody diode (i.e., a backgate diode). The body diode enables flow of anelectric current in a reverse direction of that of the original currentpath. A diode having the same functional characteristics as that of thebody diode might be optionally provided.

The power supply voltage VCC of the DC power supply BAT is supplied tothe primary winding 107 of the transformer TR by way of the NMOS 101 andthe NMOS 102. Therefore a high voltage proportional to a winding ratioof the secondary winding 108 is induced. This induced high voltage issupplied to a cold cathode fluorescent lamp FL, serving as load, so asto turn on the lamp. The PMOS 103 and the PMOS 104 as well as thecapacitors 105, 106 are functioning as suppressing occurrence of ananomalous peak overvoltage, or recovering the flyback energy.

Capacitors 111 and 112 as well as resistors 117 and 118 detect a voltageto be applied to the cold cathode fluorescent lamp FL. The thus-detectedvoltage is fed back to a controller IC 200. Resistors 114, 115 detectthe electric current flowing through the cold cathode fluorescent lampFL, and the thus-detected electric current is fed back to the controllerIC 200. The capacitor 111 is used for oscillation by its capacitancefactor and the inductance factor of the transformer TR. Parasiticcapacitance of the cold cathode fluorescent lamp FL might be alsocontributing to said oscillation. As for the reference numerals 113,116, 119, and 120, they denote diodes while the reference numerals 151,152 denote capacitors for stabilizing a power supply voltage.

The controller IC 200 has a plurality of input and output pins. Thefirst pin 1P is a changeover terminal for switching between a PWM modeand an intermittent mode (hereinafter called a “burst”) mode. Input fromthe outside to the first pin 1 p is a duty signal DUTY to be used forswitching the modes and determining the duty ratio of the burst mode. Asecond pin 2P is a connector terminal to which a capacitance for settingan oscillation frequency of a burst mode oscillator (BOSC) might beconnected. A capacitor 131 for setting purpose is connected to thesecond pin 2P, so that a triangular wave signal BCT for bursting purposeis generated at the second pin 2 p.

A third pin 3P is a terminal connector to which a capacitance to be usedfor setting an oscillation frequency of a PWM mode oscillator (OSC) isconnected. In this embodiment, a setting capacitor 132 is connected tothe third pin 3P so that a triangular wave signal CT for PWM use isdeveloped at the third pin 3P. A fourth pin 4P is a terminal connectorto which a resistor for setting an electric current to be used forrecharging the third pin 3P is connected. In this case, a settingresistor 133 is connected to the fourth pin 4P so that electric current,corresponding to an electric potential RT and the resistance value ofthe resistor-, flows through the fourth pin 4P. A fifth pin 5P is aground terminal and remains at a ground potential GND.

A sixth pin 6P is a connector terminal to which a resistor for settingan electric current to be used for recharging the third pin 3P isconnected. In this embodiment, a setting resistor 134 is connected tothe sixth pin 6P where a control operation of an internal circuitenables the resistor 134 to be connected in parallel to the settingresistor 133 or disconnected therefrom by means of. A potential SRT ofthe sixth pin 6P becomes either a ground potential GND or a potential RTof the fourth pin 4P. A seventh pin 7P is a connector terminal to whicha timer latch setting capacitance is connected such that a capacitor 135to be used for setting an operating time limit for internal circuitprotection is connected to the seventh pin 7P so that a potential SCPcorresponding to electric charge of the capacitor 135 is operated.

A current detection signal (hereinafter a “detection current”) IScorresponds to the electric current flowing into the cold cathodefluorescent lamp FL is input to a ninth pin 9P by way of a resistor 140so that the current detection signal IS is input to a first erroramplifier. An eighth pin 8P is a first error amplifier output terminal.A capacitor 136 is connected between the eighth pin 8P and a ninth pin9P. The potential of the eighth pin 8P is functioning as a feedbackvoltage FB and serves as a control voltage for PWM control purpose.Unless otherwise specified, voltages will be hereinbelow determined withreference to a ground potential.

A voltage detection signal (hereinafter called a “detection voltage”) VScorresponding to a voltage applied to the cold cathode fluorescent lampFL is input to a tenth pin 10P by way of a resistor 139 and delivered toa second error amplifier. A capacitor 137 is connected between the tenthpin 10P and the eighth pin 8P.

An eleventh pin 11P is a terminal to be used for activation and settinga start-up time. Applied to the eleventh pin 11P is a signal STBproduced by means of delaying a start-up signal ST and subjecting thethus-delayed signal to noise suppression through use of a resistor 143and a capacitor 142. A twelfth pin 12P is a terminal used for connectinga capacitance for setting a slow start. Further, a capacitor 141 isconnected between the twelfth pin 12P and the ground. A slow startvoltage SS which gradually increases at the time of startup develops inthe twelfth pin 12P.

A thirteenth pin 13P is a connector terminal for synchronizationpurpose. When the controller IC 200 is operated in conjunction withanother controller IC such as use in a system LSI, the thirteenth pin13P is connected to that controller IC. A fourteenth pin 14P is aninternal clock input/output terminal where the controller IC 200 isoperated in conjunction with another IC such as use in a system LSI, thefourteenth pin 14P is connected to that controller IC.

A fifteenth pin 15P is a ground terminal of an external FET drivecircuit. A sixteenth pin 16P is a terminal to be used for outputting agate drive signal N1 of the NMOS 102. A seventeenth pin 17P is aterminal to be used for outputting a gate drive signal N2 of the NMOS104. An eighteenth pin 18P is a terminal to be used for outputting agate drive signal P2 of the PMOS 103. A nineteenth pin 19P is a terminalto be used for outputting a gate drive signal P1 of the PMOS 101. Atwentieth pin 20P is a power terminal into which a power supply voltageVCC is input.

In FIG. 2, which shows the internal configuration of the controller IC200, an OSC block 201 produces the PWM triangular wave signal CTdetermined by means of the capacitor 132 connected to the third pin 3Pand the resistors 133, 134 connected to the fourth pin 4P. The PWMtriangular wave signal CT is supplied to a PWM comparator 214. Further,the OSC block 201 produces an internal clock signal and supplies thethus-produced clock signal to a logic block 203.

A BOSC block 202 is a circuit for oscillating a triangular wave signalfor bursting purpose. The BOS block 202 produces the burst triangularwave signal BCT to be determined by the capacitor 131 connected to thesecond pin 2P. The BCT frequency is set to become considerably lowerthan a CT frequency (a BCT frequency<a CT frequency). The analog dutysignal DUTY (a DC voltage) supplied to the first pin 1P is compared withthe triangular wave signal BCT by means of a comparator 221. An n-p-ntransistor (hereinafter abbreviated simply as “NPN”) 234 is activated bymeans of such a comparison output by way of an OR circuit 239. When thedigital duty signal DUTY (of PWM format) is supplied to the first pin1P, a resistor is connected to the second pin 2P, thereby causing theBOSC block 202 to produce a predetermined burst voltage.

A PWM control signal or the like is input to the logic block 203, and aswitch drive signal is produced in accordance with a predeterminedlogic. Gate drive signals P1, P2, N1, and N2 are applied to gates of thePMOSs 101, 103 and gates of the NMOSs 102, 104 by way of the outputblock 204.

The start-up signal ST is input to a slow start block 205. When an inputto a comparator 217; that is, the voltage STB which is gradually boostedby the capacitor 142 and the resistor 143, has exceeded the referencevoltage Vref 6, the slow start block 205 is activated by an output fromthe comparator 217. The output from the comparator 217 enables drivingof the logic block 203. Here, reference numeral 249 designates aninverting circuit. Further, a flip-flop (FF) circuit 242 is reset bymeans of the output from the comparator 217 by way of an OR circuit 243.When the start block 205 is activated, the slow start voltage SSgradually increases. The slow start voltage SS is input to the PWMcomparator 214 as a comparison input. Consequently, at the time ofstart-up, the PWM control is performed in accordance with the slow startvoltage SS.

When the input has exceeded the reference voltage Vref 5 at the time ofstart-up, the comparator 216 turns off an NMOS 246 by way of an ORcircuit 247. As a result, the resistor 134 is disconnected, therebychanging the frequency of the PWM triangular wave signal CT. An outputfrom the comparator 213 is also input to the OR circuit 247.

The detection current IS proportional to the electric current of thecold cathode fluorescent lamp FL is input to a first error amplifier211. The detection current IS is compared with the reference voltageVref 2 (e.g., 1.25 volts). By means of an output corresponding to theerror, an NPN 235 connected to a constant current source I1 iscontrolled. A collector of the NPN 235 is connected to the eighth pin8P. The potential of the node is taken as a feedback voltage FB andinput to the PWM comparator 214 as a comparison input.

The PWM comparator 214 compares the triangular wave signal CT with thefeedback voltage FB or the slow start voltage SS, whichever voltage islower, thereby producing the PWM control signal. The thus-produced PWMcontrol signal is supplied to the logic block 203 by way of an ANDcircuit 248. In a stationary state achieved after end of start-upoperation, the triangular wave signal CT is compared with the feedbackvoltage FB. A set electric current is automatically controlled so as toflow into the cold cathode fluorescent lamp FL.

The capacitor 136 is connected between the eighth pin 8P and the ninthpin 9P, and the feedback voltage FB smoothly increases or decreases.Consequently, PWM control operation is smoothly performed withoutoccurrence of shock.

The detection voltage VS proportional to the voltage of the cold cathodefluorescent lamp FL is input to a second error amplifier 212 andcompared with a reference voltage Vref 3 (e.g., 1.25 volts). By means ofa resultant output corresponding to the error, an NPN 238 of a doublecollector structure whose one collector is connected to the constantcurrent source I1 is controlled. The collector of the NPN 238 is alsoconnected to the eighth pin 8P. Hence, the feedback voltage FB is alsocontrolled by means of the detection voltage VS. When the feedbackvoltage FB has exceeded the reference voltage Vref1 (e.g., 3 volts), ap-n-p transistor (hereinafter referred to simply as “PNP”) 231 turns on,thereby limiting an excessive rise in the feedback voltage FB.

A comparator 215 compares a voltage into which the power supply voltageVCC has been divided by means of resistors 240, 241 with a referencevoltage Vref 7 (e.g., 2.2 volts). At a point in time when the powersupply voltage VCC has reached a predetermined value, the output isinverted, thus resetting the FF circuit 242 by way of the OR circuit243.

A comparator 218 compares the slow start voltage SS with a referencevoltage Vref 8 (e.g., 2.2 volts). When the voltage SS has becomegreater, the NPN 234 is activated by way of an AND circuit 244 and theOR circuit 239. As a result of the NPN 234 being turned on, the diode232 is reversely biased by means of a current source I2. Consequently,standard operation of the first error amplifier 211 can be made.

When the NPN 238, whose remaining collector is connected to a constantcurrent source I3, is turned on by means of the second error amplifier212, the voltage of the comparator 219 becomes lower than a referencevoltage Vref 9 (e.g., 3.0 volts), and a comparison output from thecomparator 219 is inverted. A comparator 220 compares the feedbackvoltage FB with a reference voltage Vref 10 (e.g., 3.0 volts) so that incase of the feedback voltage FB coming to be higher, a comparison outputis inverted. The inverted signals output from the comparator 219 and thecomparator 220 and the signal output from the comparator 218 are appliedto a timer block 206 by way of an OR circuit 245. After a predeterminedperiod of time has been connected, an output is produced by the timerblock 206, whereby the FF circuit 242 is set, and operation of the logicblock 203 is stopped by a Q output from the FF circuit 242.

Operation of the inverter having the foregoing configuration will now bedescribed in connection with PWM control and burst control.

During a period in which the duty signal DUTY has exceeded the bursttriangular wave signal BCT (i.e., an ON-DUTY period), PWM control isperformed. On the other hand, during a period in which the duty signalDUTY is lower than the burst triangular wave signal BCT (i.e., anOFF-DUTY period), PWM control is halted, and power supply to the coldcathode fluorescent lamp FL is suspended.

Assuming that the frequency of the PWM triangular wave signal CT assumesis 50 kHz, and the PWM triangular wave signal CT is subjected to burstcontrol performed by the triangular wave signal BCT of, e.g., 150 Hz,this might not cause any visual problem. Moreover, by means ofcontrolling the magnitude of the duty signal DUTY, control of the powersupply, in other words, control of light amount, can be performed in theexcess range more than where the power can be supplied to the coldcathode fluorescent lamp FL by relying on only PWM control.

As for a specific circuit operation which is performed during a periodin which the duty signal DUTY is below the burst triangular wave signalBCT, an output from the comparator 221 remains at a low (L) level andthe NPN transistor 234 remains in off-state.

As a result, the diode 232 is forwardly biased by means of the constantcurrent source I2 so that an input to the first error amplifier 211becomes a highlevel, and the NPN transistor 235 becomes on-state, inwhich the feedback voltage FB is regulated to a low voltage.

The PWM comparator 214 compares a lower one of two negative inputvoltages with a positive triangular wave signal CT. In this case, thePWM control signal is not output.

Next, when the duty signal DUTY has surpassed the burst triangular wavesignal BCT, the NPN transistor 234 becomes on-state, and the diode 232is reversely biased to become off-state. At this time, since thedetection voltage IS remains at a low level, the first error amplifier211 produces an output corresponding to the input detection voltage ISso that controlling the conduction degree of the NPN transistor 235 canbe controlled. The collector voltage of the NPN transistor 235, namely,the feedback voltage FB, gradually increases by means of operation ofthe capacitor 136 connected between the eighth and ninth pins, andreaches a stationary state corresponding to original feedback. As aresult, the PWM control signal is supplied from the PWM comparator 214to the logic block 203, whereby the gate drive signals P1 to N2 areoutput and the NMOSs 101, 102 are subjected to PWM control.Synchronously, the PMOSs 103, 104 can be controlled.

A logic used for producing the gate drive signals P1 to N2 in the logicblock 203 and the output block 204 at the time of PWM control operationwill now be described in detail by reference to the timing chart of afirst example shown in FIG. 3 and operating states achieved at timingsshown in FIG. 4. Operation of the invention will be described byreference to a waveform chart shown in FIG. 5.

On the basis of the pulse width modulation signal, namely, the PWMtriangular wave signal CT and the feedback voltage FB, the output block204 outputs a first gate drive signal N1 to be used for driving the NMOS101 serving as the first semiconductor switch; a second gate drivesignal N2 to be used for driving the NMOS 102 serving as the secondsemiconductor switch; a third gate drive signal P1 to be used fordriving the PMOS 103 serving as the third semiconductor switch; and afourth gate drive signal P2 to be used for driving the PMOS 104 servingas the fourth semiconductor switch.

The gate drive signals P1, P2, N1, and N2 are produced at the timings ofan off-state period where all the NMOS 101, NMOS 102, the PMOS 103, andthe PMOS 104 are turned off, said timings are produced during theinterval periods such that a first semiconductor switch group,consisting of the NMOS 101 and the PMOS 103, and a second semiconductorswitch group, consisting of the NMOS 102 and the PMOS 104, arealternately made on-state synchronously with triangular wave signals ofthe triangular wave signal train CT.

More specifically, the NOMS 101 and the PMOS 103 are turned to theon-state at every other peak of the triangular wave signal of thetriangular wave signal train of which peaks appear in a range lower thanthe feedback signal FB. These on-state NMOS 101 and the PMOS 103continuously remain the state until the immediately-subsequenttriangular wave signal becomes equal to the feedback signal FB. On theother hand, the NMOS 102 and the PMOS 104 are turned on at every otherpeak of the triangular wave signal of which peaks appearin the rangelower than the feedback signal FB, but said peaks are different from thepeaks at which the NMOS 101 and the PMOS 103 of the triangular wavesignal train CT become on-states. The NMOS S102 and the PMOS 104continuously remain activated until the immediately-subsequenttriangular wave signal becomes equal to the feedback signal FB.

In a duration “i” shown in FIG. 3, the gate drive signal N1 is at a highlevel, and the NMOS 101 is on-state, and an electric current flows fromthe DC power source BAT (i.e., a power supply voltage VCC) to theprimary winding 107 by way of the center tap T and the first terminal Ain the first direction. This state is shown in FIG. 4A. The gate drivesignal P1 remains at a low level, and the PMOS 103 becomes on-state.Consequently, a current loop is established from the center T with theother end terminal B (hereinafter called a second terminal) of theprimary winding 107, the first capacitor C1, and the PMOS 103. Anelectric current flows in the direction illustrated by the arrows. Here,the voltage of the first terminal A (hereinafter called a voltage atpoint A) Va corresponds to the ground voltage GND.

During a duration ii, the gate drive signal N1 becomes low, and the gatedrive signal P1 becomes high, whereupon the NMOS 101, the NMOS 102, thePMOS 103, and the PMOS 104 all become off-state. FIG. 4B shows thisstate. During the duration ii, the electric current running in the firstdirection flows into the DC power source BAT (i.e., the power supplyvoltage VCC) by way of the body diode of the NMOS 102, the secondterminal B, and the center tap T, which is caused by the energy storedin the transformer TR. Similarly, the electric current flows through acurrent loop consisting of the second capacitor 106, the body diode ofthe PMOS 104, the center tap T, and the first terminal A, due to theenergy stored in the transformer TR. In this state, the voltage Vaobtained at the point A becomes a voltage defined by summation of avoltage drop Vf due to the body diode and twice of the power supplyvoltage VCC; that is, 2×VCC+Vf.

In a latter half the duration ii, when the electric current running inthe first direction due to the energy stored in the transformer TR hasbecome zero, the voltage Va obtained at the point A becomes the powersupply voltage VCC as indicated by broken lines in a duration ii′. Sucha state of the electric current of being zero might be produced when theduty factor of the pulse width modulation signal is made small. In otherwords, it might be produced when the on-state period of the gate drivesignals P1, P2, N1, and N2 is short. In this case, the state in that theelectric current flowing through the primary winding 107 becomes zero isrealized before the direction of the electric current flowing throughthe primary winding 107 is switched.

In a duration iii, the gate drive signal N2 is high, and the NMOS 102 ison-state so that an electric current flows from the DC power source BAT(the power supply voltage VCC), to the primary winding 107 in the seconddirection by way of the center tap T and the second terminal B. FIG. 4Cshows this state. The gate drive signal P2 remains at a low level, andthe PMOS 104 becomes on-state. Hence, a current loop is formed so as tooriginate from the center tap T by way of the first terminal A of theprimary winding 107, the second capacitor 106, and the PMOS 104,whereupon the electric current flows in the direction indicated by thearrows. The voltage Va obtained at the point A is twice the power supplyvoltage VCC (i.e., 2×VCC).

In a duration iv, the gate drive signal N2 becomes low, and the gatedrive signal P2 becomes high where the NMOS 101, the NMOS 102, the PMOS103, and the PMOS 104 all become inoperative. FIG. 4D shows this state.In the duration iv, the electric current running in the second directionflows into the DC power source BAT (i.e., the power supply voltage VCC)by way of the body diode of the NMOS 101, the first terminal A, and thecenter tap T which is caused by the energy stored in the transformer TR.Similarly, the electric current flows through a current loop consistingof the first capacitor 105, the body diode of the PMOS 103, the centertap T, and the second terminal B, due to the energy stored in thetransformer TR. In this state, the voltage Va obtained at the point Abecomes a voltage −Vf, which is lower than the ground voltage by anamount corresponding to the voltage drop Vf due to the body diode.

In a latter half of the duration ii, when the electric current, runningin the second direction due the energy stored in the transformer TR, hasbecome zero, the voltage Va obtained at the point A becomes the powersupply voltage VCC as indicated by broken lines in a duration iv′, as inthe case of the duration ii′. The voltage obtained at point B of thesecond terminal B becomes reverse to the voltage Va obtained at point Aduring the durations “i” to iv.

FIG. 5 shows a measured characteristic of the voltage Va obtained at thepoint A and a measured characteristic of the load current Io flowinginto the cold cathode fluorescent lamp FL. FIG. 5A shows the voltage Vaobtained at the point A and the load current Io of the first embodiment.FIG. 5B shows the voltage Va obtained at the point A and the loadcurrent Io, both being obtained when the circuit shown in FIG. 1 is notprovided with a series circuit consisting of the PMOS 103 and the firstcapacitor 105 and a series circuit consisting of the PMOS 104 and thesecond capacitor 106.

In FIG. 5A, showing the characteristic of the invention, an upper limitof the voltage Va obtained at the point A is defined as a voltageobtained by adding the voltage drop Vf due to the body diode to doublethe power supply voltage VCC (2×VCC+Vf). In contrast, in the case ofFIG. 5B, showing a characteristic obtained when the series circuitsconsisting of the PMOSs 103, 104 and the capacitors 105, 106 are notprovided, an abnormal voltage Vpeak arises in the form of a very highspike in the voltage Va obtained at point A when the voltage has enteredthe duration ii. The abnormal voltage V peak is measured to be aboutfive to six times the power supply voltage VCC.

When the abnormal voltage Vpeak arises, an element of high resistancedesign, which is capable of withstanding the abnormal voltage Vpeak,must be used. Further, the abnormal voltage Vpeak will act as a noisesource for the surroundings.

According to the invention, the series circuits consisting of the PMOS103, the PMOS 104, and the respective capacitors 105, 106 are providedbetween the center tap T of the primary winding 107 and the terminals A,B. The PMOSs 103, 104 are turned on synchronously with the NMOSs 101,102 with a predetermined relationship. Thus, fine adjustment of thepower supply to the load can be made with a simple configuration.Occurrence of an abnormal high voltage, which would otherwise be causedat the time of switching, can be prevented, and an inverter can beconfigured with use of circuit elements of low withstand voltage.Recovery of flyback energy is also performed, so that a power conversionefficiency is also improved.

FIG. 6 is a timing chart of a second example for the inverter of thefirst embodiment shown in FIG. 1, wherein the on-state timings of thePMOSs 103, 104 are modified differently from those of the timing chartshown in FIG. 3. FIG. 7 is a view showing operating states of theinverter achieved at respective timings shown in FIG. 6.

FIG. 6 is similar to FIG. 3, except for the on-state timings of thePMOSs 103, 104, which are different from those shown in FIG. 3.

In FIG. 6, the NMOS 101 to be driven by the gate drive signal N1 isturned-on at every other peak point of the triangular wave signal of thetriangular wave signal train CT. The NMOS 101 continues to be activeuntil an immediately-subsequent triangular signal becomes equal to thefeedback signal FB.

The NMOS 102 to be driven by the gate drive signal N2 is turned-on atevery other peak point of the triangular wave signal of the triangularwave signal train CT, the peak points being differed from those of thetriangular wave signal at which the NMOS 101 is turned-on. The NMOS 102remains continuously on-state until an immediately-subsequent triangularsignal becomes equal to the feedback signal FB.

The PMOS 103 is turned on faster than the NMOS 101 and after lapse of apredetermined time Toff since the NMOS 102 finished its on-state. ThePMOS 103 remains active for a period during which the NMOS 101 ison-state.

The PMOS 104 is turned on faster than the NMOS 102 and after lapse of apredetermined time Toff since the NMOS 101 finished becoming on-state.The PMOS 104 remains on-state for a period during the period of the NMOS102 being on-state.

The predetermined time Toff is provided for ensuring a period during theperiod of the NMOS 101, the NMOS 102, the PMOS 103, and the PMOS 104being kept under off-state. During the period ii, the PMOS 104 is turnedon after lapse of the predetermined time Toff. While in the duration iv,the PMOS 103 is turned on after lapse of the predetermined time Toff.

The states illustrated in FIGS. 7B and 7D show that the PMOSs 103, 104kept under off-state during the period Toff achieved at the initialphase, and the electric current flows by way of the body diode. ThePMOSs 103, 104 becomes on-state after the period Toff.

By turning on the PMOSs 103, 104 prior to the NMOSs 101, 102 beingon-state, the period of the electric current flowing through the bodydiodes of the PMOSs 103, 104 can be made shortened, and the losscorresponding to the voltage drop Vf can be diminished.

FIGS. 8 and 9 are diagrams showing another embodiment in which theconfiguration of the main circuit of the inverter shown in FIG. 1 ischanged.

FIG. 8 shows a sequence of connection of the series circuit consistingof the PMOS 103 functioning as the third semiconductor switch and thefirst capacitor 105 and a sequence of connection of the series circuitconsisting of the PMOS 104 functioning as the fourth semiconductorswitch and the second capacitor 106, in the manner inverted to thatshown in FIG. 1. Inverter operation is performed similarly by means ofthe main circuit configuration shown in FIG. 8.

FIG. 9 shows in that an NMOS is used for the third semiconductor switch103 and the fourth semiconductor switch 104, both being shown in FIG. 8.By means of such a configuration, the first through fourth semiconductorswitches 101 to 104 can be embodied all by the NMOSs. Hence, there canbe constituted an inverter which is more advantageous in terms of costand area. In association with the third and fourth semiconductorswitches 103, 104 being embodied as NMOSs, the inverter control IC 200supplies the gate drive signals N3, N4 to be used for appropriatecontrol of the third and fourth semiconductor switches 103, 104 at thesame timings as those shown in FIGS. 3 and 6.

FIG. 10 shows that the center tap T of the transformer TR is connectedto the ground serving as a common potential point. The DC power supplyvoltage VCC supplied from the battery power source BAT is connected tothe first terminal A by way of the NMOS 101 functioning as the firstsemiconductor switch, so as to form a current path in the firstdirection toward the primary winding 107 of the transformer TR. The DCpower supply voltage VCC is connected to the second terminal B by way ofthe NMOS 102 functioning as the second semiconductor switch, therebyforming a current path in the second direction toward the primarywinding 107 of the transformer TR.

A series circuit consisting of the first capacitor 105 and the NMOS 103functioning as the third semiconductor switch is connected between thecenter tap T of the primary winding 107 and the second terminal B.Similarly, a series circuit consisting of the second capacitor 106 andthe NMOS 104 serving as the fourth semiconductor switch is connectedbetween the center tap T of the primary winding 107 and the firstterminal A.

The first through fourth semiconductor switches 101 to 104 are turned onand off in the same manner as that of the semiconductor switches 101through 104 shown in FIG. 1. In FIG. 10, the semiconductor switches 101,102 can be embodied as PMOSs.

Even in the case shown in FIG. 10, supplied from the inverter control IC200 are the gate drive signals N3, N4 to be used for appropriate controlof the first through fourth semiconductor switches 101 to 104 at thesame timings as those shown in FIGS. 3 and 6.

The foregoing descriptions have described use of MOSFETs as the firstthrough fourth semiconductor switches. The first through fourthsemiconductor switches may be embodied by devices which can be turned onand off by means of a drive signal with body diodes. Alternatively, theswitches may be provided with diodes having the same functions as thoseof the body diodes.

According to the invention, a transformer having a primary winding witha center tap to which a DC power supply voltage is supplied is used foran inverter which produces from a DC power source an AC voltage to beused for activating a load. First and second semiconductor switches tobe activated alternately are provided between ends of the primarywinding and a common potential point. An electric current flowing intoload is fed back, thereby subjecting semiconductor switches to pulsewidth modulation (PWM) control. Power supply to the load can becontrolled elaborately with a simple configuration.

A series circuit consisting of a first capacitor and a thirdsemiconductor switch and another series circuit consisting of a secondcapacitor and a fourth semiconductor switch are connected between thecenter tap of the primary winding and ends of the primary winding. Thethird and fourth semiconductor switches are activated in synchronismwith the first and second semiconductor switches, thereby preventingoccurrence of an anomalous high voltage, which would otherwise be causedat the time of switching. As a result, the inverter can be configuredthrough use of a circuit element of low withstand voltage.

The semiconductor switch is embodied as a MOS transistor, so thatoperation of the body diode (i.e., a backgate diode) can be utilizedeffectively.

The third and fourth semiconductor switches are activated prior toactivation of the first and second semiconductor switches. A periodduring which the electric current flows through the body diodes of thesemiconductor switches is shortened, thereby diminishing a losscorresponding to the amount of voltage loss.

Burst control which enables adjustment of a ratio of an ON time to anOFF time is performed in conjunction with pulse width modulationcontrol. Power supply to the load can be adjusted over a wide range inexcess of the limit of the pulse width modulation control.

1. A DC-AC converter comprising: a transformer having a primary windingwith a center tap, and at least one secondary winding, in which thecenter tap is connected to a first potential point of a DC power source;a first semiconductor switch which is connected between one end of theprimary winding and a second potential point of the DC power source andcauses an electric current to flow through the primary winding in afirst direction; a second semiconductor switch which is connectedbetween the other end of the primary winding and the second potentialpoint and causes an electric current to flow through the primary windingin a second direction; a first capacitor and a third semiconductorswitch, which are connected in series between the other end of theprimary winding and the center tap; a second capacitor and a fourthsemiconductor switch, which are connected in series between the one endof the primary winding and the center tap; a current detection circuitto be used for detecting an electric current which flows into a loadconnected to the secondary winding; a logic circuit to be used foroutputting a switch drive signal and which produces, on the basis of theelectric current flowing through the load, a first switch drive signalto be used for turning on the first semiconductor switch, a secondswitch drive signal to be used for turning on the second semiconductorswitch, a third switch drive signal to be used for turning on the thirdsemiconductor switch, and a fourth switch drive signal to be used forturning on the fourth semiconductor switch, wherein the first throughfourth switch drive signals are produced at timings of an off-stateperiod where the first through fourth semiconductor switches are turnedoff, said timings being produced during interval periods such that afirst semiconductor switch group, comprising the first semiconductorswitch and the third semiconductor switch, and a second semiconductorswitch group, comprising the second semiconductor switch and the fourthsemiconductor switch, are alternately made on-state synchronously withtriangular wave signals of the triangular wave signal train.
 2. TheDC-AC converter according to claim 1, wherein the first and thirdsemiconductor switches are turned on at every other peak of thetriangular wave signal of the triangular wave signal train and remainon-sate until an immediately-subsequent triangular wave signal becomesequal to the feedback signal; and further wherein the second and fourthsemiconductor switches are turned on at every other peak of thetriangular wave signal of the triangular wave signal train, which isdifferent from that of being generated when the first and thirdsemiconductor switches are turned on, and remain on-state until animmediately-subsequent triangular wave signal becomes equal to thefeedback signal.
 3. The DC-AC converter according to claim 1, whereinthe first semiconductor switch is turned on at every other peak of thetriangular wave signal of the triangular wave signal train and remainson-state until an immediately-subsequent triangular wave signal becomesequal to the feedback signal; the second semiconductor switch is turnedon at every other peak of the triangular wave signal of the triangularwave signal train, which is different from that of being generated whenthe first semiconductor switch is turned on, and remains on-state untilan immediately-subsequent triangular wave signal becomes equal to thefeedback signal; the third semiconductor switch is turned on at a timingbefore the first semiconductor switch is turned on, but after lapse of apredetermined period of time since the second semiconductor switchcompletes the on-sate period thereof and remains on-state during aperiod in which the first semiconductor switch remains on-state; and thefourth semiconductor switch is turned-on at a timing before the secondsemiconductor switch is turned on, but after lapse of a predeterminedperiod of time since the first semiconductor switch completes theon-state period thereof and remains on-state during a period in whichthe second semiconductor switch remains on-state.
 4. The DC-AC converteraccording to claim 1, wherein the first through fourth semiconductorswitches are MOS field-effect transistors.
 5. The DC-AC converteraccording to claim 1, wherein a burst control signal in the form of apulse train, which allows a ratio of on-state period and off-stateperiod to be controlled, whereby the first through fourth switch drivesignals can be produced or stopped.
 6. (canceled)